Studies in other laboratories have shown that fusion of malignant cell lines with intraspecific normal cells often results in suppression of tumorigenicity in stable hybrids. This suppression is sometimes transient, leading to speculation that the normal cell contributes genes that suppress malignancy, and when these genes are lost by further passage in tissue culture, malignancy is reexpressed as a recessive trait. We have recently begun to study the expression of malignancy in hybrids of malignant and normal cells of human bronchial epithelial origin (cells derived from the same tissue as well as the same species).